9. Hostage Negotiation - Persona 5
Persona 5 utilized a feature called Hostage Negotiation in which any of your characters could be 'taken hostage' by an enemy who knocked them down with an elemental attack they were weak to.
Once your comrade was the enemy's hostage you could either pay the enemy to release them, talk to them and convince them to let your cohort go, or do nothing – in which case your comrade would be insta-killed.
No other major game release has used a mechanic like this – and the worst part is that Persona 5 barely used it. The Hostage mechanic wouldn't trigger during boss battles and it has such a rare chance to trigger in regular battles that many players never even saw it occur.
Imagine if the feature was expanded a bit in other games? Fire Emblem, but when a character is defeated you have to pay up or negotiate in a dialogue event to get them released or else they're perma-dead? A competitive FPS where you can offer to pay the other team a game resource if you're beaten in order to get a faster respawn? Or even just using it more in similar RPG-style games like the next Persona or a Final Fantasy title.